1. Field of the Invention
The present invention relates to a method for providing a multimedia broadcast/multicast service (MBMS) of a universal mobile telecommunications system (UMTS), and more particularly, to a method for transmitting multicast data through a downlink shared channel.
2. Description of the Background Art
The developments in wireless mobile communications have lead users to favor using mobile phones rather than wired telephones. However, for services providing a large quantity of data, for example an amount above that generally provided by voice communications, to mobile phones through a wireless access network, the performance of mobile communication systems cannot match that of existing wired communication systems. Accordingly, technical developments for IMT-2000, a communication system allowing high capacity data communications, have been made and standardization of the technology is being actively pursued among various companies and organizations.
A universal mobile telecommunications system (UMTS) is a third generation mobile communication system that has evolved from a standard known as Global System for Mobile communications (GSM). This standard is a European standard which aims to provide an improved mobile communication service based on a GSM core network and wideband code division multiple access (W-CDMA) technology.
In December 1998, the ETSI of Europe, the ARIB/TTC of Japan, the T1 of the United States, and the TTA of Korea formed a Third Generation Partnership Project (3GPP). The 3GPP is creating detailed specifications for the UMTS technology. In order to achieve rapid and efficient technical development of the UMTS, five technical specification groups (TSG) have been created within the 3GPP for performing the standardization of the UMTS by considering the independent nature of the network elements and their operations.
Each TSG develops, approves, and manages the standard specification within a related region. Among these groups, the radio access network (RAN) group (TSG-RAN) develops the standards for the functions, requirements, and interface of the UMTS terrestrial radio access network (UTRAN), which is a new radio access network for supporting W-CDMA access technology in the UMTS.
FIG. 1 shows a network structure of a general UMTS.
As shown in FIG. 1, the UMTS is roughly divided into a terminal (UE user equipment), a UTRAN and a core network.
The UTRAN includes one or more radio network sub-systems (RNS). Each RNS includes an RNC and one or more Node Bs managed by the RNCs.
Node Bs are managed by the RNCs, receive information sent by the physical layer of a terminal (e.g., mobile station, user equipment and/or subscriber unit) through an uplink, and transmit data to a terminal through a downlink. Node Bs, thus, operate as access points of the UTRAN for a terminal.
The RNCs perform functions which include assigning and managing radio resources, and operate as an access point with respect to the core network.
A primary function of the UTRAN is constructing and maintaining a radio access bearer (RAB) for a call connection between the terminal and the core network. The core network applies quality of service (QoS) requirements of end-to-end to the RAB, and accordingly, the UTRAN can satisfy the QoS requirements of the end-to-end by constructing and maintaining the RAB.
The RAB service is divided into an lu bearer service and a radio bearer service of a lower concept. The lu bearer service handles reliable user data transmission between boundary nodes of UTRAN and the core network, while the radio bearer service handles reliable user data transmission between the terminal and UTRAN.
The core network includes a mobile switching center (MSC) and a gateway mobile switching center (GMSC) connected together for supporting a circuit switched (CS) service. The core network also includes a serving GPRS support node (SGSN) and a gateway GPRS support node connected together for supporting a packet switched (PS) service.
The services provided to a specific terminal are roughly divided into the circuit switched (CS) services and the packet switched (PS) services. For example, a general voice conversation service is a circuit switched service, while a Web browsing service via an Internet connection is classified as a packet switched (PS) service.
For supporting circuit switched services, the RNCs are connected to the MSC of the core network and the MSC is connected to the GMSC that manages the connection with other networks. For supporting packet switched services, the RNCs are connected to the SGSN and the GGSN of the core network. The SGSN supports packet communications with the RNCs and the GGSN manages the connection with other packet switched networks, such as the Internet.
Various types of interfaces exist between network components to allow the network components to transmit and receive information with each other. An interface between the RNC and the core network is defined as an lu interface. In particular, the lu interface between the RNCs and the core network for packet switched systems is defined as “lu-PS” and the lu interface between the RNCs and the core network for circuit switched systems is defined as “lu-CS.”
A radio network temporary identifier (RNTI) is used to identify a terminal while connection between the terminal and the UTRAN is maintained. Four RNTIs are defined; S-RNTI, D-RNTI, C-RNTI and U-RNTI.
The S-RNTI (Serving RNC RNTI) is assigned by an SRNC (Serving RNC) when a connection between a terminal and UTRAN is set. The S-RNTI is information by which the SRNC may identify a corresponding terminal.
The D-RNTI (Drift RNC RNTI) is assigned by a DRNC (Drift RNC) when a handover occurs between RNCs according to movement of a terminal. The D-RNTI is information by which the DRNC may identify a corresponding terminal.
The C-RNTI (Cell RNTI) is information by which a terminal may be identified in a CRNC (Controlling RNC). When a terminal enters a new cell, it is assigned a new C-RNTI value by the CRNC.
The U-RNTI (UTRAN RNTI) includes an SRNC identity and an S-RNTI. Since the SRNC and a terminal in the SRNC may be identified, it may be said that the U-RNTI provides absolute identification information.
When data is transmitted via a common transport channel, a MAC-c/sh entity adds the C-RNTI or the U-RNTI to a header of a MAC PDU which is then transmitted. A UE ID type indicator, which indicates a type of the RNTI added in the header of the MAC PDU, is also added to the header.
FIG. 2 illustrates a radio protocol between the terminal and the UTRAN on the basis of the 3GPP wireless access network standards.
With reference to FIG. 2, the radio access interface protocol includes horizontal layers comprising a physical layer, a data link layer and a network layer, and vertical planes comprising a user plane for transmitting data information and a control plane for transmitting control signals.
The user plane is a region to which traffic information of a user such as voice or an IP packet is transmitted. The control plane is a region to which control information such as an interface of a network or maintenance and management of a call is transmitted.
In FIG. 2, protocol layers can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model well known in the art of communication systems.
The first layer (PHY) provides an information transfer service to the upper layer by using various radio transfer techniques.
The first layer is connected to the MAC layer through a transport channel, and data is transferred between the MAC layer and the PHY layer through the transport channel.
Data is transmitted according to transmission time interval (TTI) through Jo the transport channel. The physical channel transfers data by dividing it by the unit of certain time called a frame. In order to synchronize the transport channel between the UE and UTRAN, a connection frame number (CFN) is used. The CFN value has the range of 0˜255 in case of transport channels except for a paging channel (PCH). That is, CFN is repeatedly circulated by the period of 256 frames.
Besides the CFN, a system frame number (SFN) is also used to synchronize the physical channel. The SFN value has the range of 0˜4095 and repeated by the period of 4096 frames.
The second layer (L2) includes a MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer.
The MAC layer provides a re-allocation service of the MAC parameter for allocation and re-allocation of radio resources.
The MAC layer is connected to the radio link control (RLC) layer (which is an upper layer) through a logical channel, and various logical channels are provided according to the kind of transmitted information. In general, when information of the control plane is transmitted, a control channel is used. When information of the user plane is transmitted, a traffic channel is used.
The MAC is classified into an MAC-b sublayer, an MAC-d sublayer and an MAC-c/sh sublayer according to types of managed transport channels. The MAC-b sublayer manages a BCH (Broadcast Channel) handling broadcast of system information, while the MAC-c/sh sublayer manages shared transport channel such as FACH (Forward Access Channel), DSCH (Downlink Shared Channel), or the like, shared with other terminals.
In UTRAN, the MAC-c/sh sublayer is positioned at a control RNC (CRNC) and manages channels shared by every terminal in a cell, so that one MAC-c/sh sublayer exists in each cell.
The MAC-d sublayer manages a DCH (Dedicated Channel), a dedicated transport channel for a specific terminal. Accordingly, the MAC-d sublayer is positioned at a serving RNC (SRNC) managing a corresponding terminal, and one MAC-d sublayer exists also at each terminal.
A radio link control (RLC) layer supports a reliable data transmission and may perform a function of segmentation and concatenation of an RLC service data unit (SDU) coming from a higher layer. The RLC SDU transferred from the higher layer is adjusted in its size according to a throughput capacity at the RLC layer, to which header information is added, and then transferred in a form of a PDU (Protocol Data Unit) to the MAC layer. The RLC layer includes an RLC buffer for storing the RLC SDU or the RLC PDU coming from the higher layer.
A broadcast/multicast control (BMC) layer performs functions of scheduling a cell broadcast message (CB) transferred from the core network and broadcasting the CB to UEs positioned in a specific cell(s). At the side of UTRAN, the CB message transferred from the upper layer is combined with information, such as a message ID, a serial number or a coding scheme, and transferred in a form of BMC message to the RLC layer and to the MAC layer through a CTCH (Common Traffic Channel), a logical channel. In this case, the logical channel CTCH is mapped to a FACH (Forward Access Channel), a transport channel, and an S-CCPCH (Secondary Common Control Physical Channel), a physical channel.
A packet data convergence protocol (PDCP) layer is an upper layer of the RLC layer, allowing data to be transmitted effectively on a radio interface with a relatively small bandwidth through a network protocol such as the IPv4 or the IPv6. For this purpose, the PDCP layer performs a function of reducing unnecessary control information, which is called a header compression, and in this respect, RFC2507 and RFC3095 (robust header compression: ROHC), a header compression technique defined by an Internet standardization group called an IETF (Internet Engineering Task Force), can be used. In these methods, because the only information requisite for the header part of a data is transmitted, control information is transmitted, so that an amount of data transmission can be reduced.
The RRC layer positioned in the lowest portion of the third layer (L3) is defined only in the control plane and controls the logical channels, the transport channels, and the physical channels in relation to the setup, the reconfiguration, and the release of the RBs. The RB signifies a service provided by the second layer for data transmission between the terminal and UTRAN, and setting up the RB means processes of stipulating the characteristics of a protocol layer and a channel, which are required for providing a specific service, and setting the respective detailed parameters and operation methods.
The RLC layer may belong to the user plane or to the control plane depending upon the type of layer connected at the upper layer of the RLC layer. If the RLC layer receives data from the RRC layer, the RLC layer belongs to the control plane. Otherwise, the RLC layer belongs to the user plane.
As shown in FIG. 2, there may be several entities in one RLC layer or one PDCP layer layer. More than one layer may be present because one terminal generally has a plurality of RBs and only one RLC entity and only one PDCP entity are used for one RB.
The MAC-sublayer will now be described.
A primary function of the MAC layer existing between the RLC and the physical layer is mapping the logical channel and the transport channel. The reason is because channel processing methods of the upper layer and the lower layer of the MAC are different. That is, at the upper layer of the MAC, data is processed separately by using the control channel of the control plane and the traffic channel of the user plane according to the content of the data that the channel transmits. Meanwhile, at the lower layer, data is processed separately by using a common channel and a dedicated channel depending on whether a channel is shared, so inter-channel mapping is important.
FIG. 3 illustrates mapping relations between the logical channel and the transport channel at the UE. In case of UTRAN, the directions of arrows are the opposite.
Another important function of the MAC layer may be a logical channel multiplexing. The MAC maps several logical channels to one transport channel to obtain a multiplexing gain which heightens the efficiency of the transport channel. Such multiplexing may provide much higher gain for data transmitted intermittently and packet data. Therefore, the multiplexing function is used for an SRB (Signaling Radio Bearer) or a packet service (PS) RAB. Because data is continuously transmitted in a circuit service (CS) RAB, the multiplexing function is not used. The SRB is an RB used specifically for exchanging an RRC message or an NAS message between the terminal and the UTRAN.
Accordingly, the MAC provides a flexibility in channel selection and an efficiency of a channel resource through the channel mapping and logical channel multiplexing. In this case, in order to support the channel mapping and the logical channel multiplexing, additional functions are required. That is, four functions are additionally performed in the MAC.
1. Priority Handling
In order to support various channel mapping structures, the MAC performs a priority handling function. The priority handling includes two types: one is priority handling among several UEs, and the other is priority handling for one UE.
The priority handling among UEs corresponds to a case that data of several UEs are transmitted at the downlink through a common transport channel (FACH or DSCH). In this case, the MAC first transmits data of a UE with a higher priority. That is, the MAC suitably allocates the common channel to each UE at each transmission time interval (TTI), to thereby heighten an efficiency of the channel resource. This is related to a dynamic scheduling function.
A priority handling on one UE corresponds to a case that several logical channels belonging to one UE is mapped to one transport channel. The MAC determines a priority from the logical channel priority. This is related to a transport format combination selection, and the MAC selects a transport format combination that can first transmit data of a logical channel with a higher priority.
2. Transport Format Combination Selection
The MAC transmits transport blocks (TB) to the physical layer through the transport channel. The transport format (TF) means a regulation for a size and the number of TBs that one transport channel transmits. In determining the TF for a specific transport channel, the MAC should even consider the transport channel multiplexing in the physical layer.
The transport channel multiplexing refers to mapping plural transport channels to one coded composite transport channel (CCTrCH). Although this function is performed in the physical layer, the MAC should consider every transport channel mapped to the same CCTrCH in determining the TF. Actually, the amount of data processed in the physical layer is the amount of data transmitted through CCTrCH, so the MAC should determine the TF of each transport channel in consideration of CCTrCH. In this case, a combination of TF is called a transport format combination (TFC). The TFC is not determined by the MAC itself but selected from an available TFC set (TFCS) that the RRC layer informs. That is, the RRC informs the MAC of an available TFCS for one CCTrCH in an initial setting, and then the MAC selects a suitable TFC from the TFCS at each TTI.
Selection of a suitable TFC from a given TFCS at each TTI is a function performed by the MAC, which includes two steps.
First, the MAC constitutes a valid TFC set in the TFCS assigned to CCTrCH, and selects an appropriate TFC in the valid TFC set. The valid TFC set is a set of TFCs actually available for a corresponding TTI among assigned TFCS. The selection of a suitable TFC is take into account a channel environment changing at every moment. When a TFC is selected to be used in the corresponding TTI in the valid TFC set, the MAC selects a TFC on the basis of a priority of the logical channel. That is, the MAC selects a TFC that can transmit preferentially data of the logical channel with a higher priority, and such TFC selection is related to the priority processing function.
As for the RACH or CPCH, the common transport channel of the uplink, because one transport channel constitutes one CCTrCH, the term of the TF selection is used for the channels.
3. Identification
The MAC requires an identification function. The reason is because, first, the common transport channel is shared for use by several UEs, so each UE needs to be identified, and second, each logical channel needs to be identified due to the logical channel multiplexing. Accordingly, the MAC inserts four types of fields into a header of the MAC PDU for identification as shown in FIG. 4. The fields of the MAC header do not necessarily exist, and their existence is determined depending on a mapping relation of the logical channel and the transport channel.
The identification of the terminal is required when the dedicated logical channel such as DCCH or DTCH is mapped to a common transport channel such as RACH, FACH, CPCH (Control Physical Channel), DSCH or USCH (Uplink Shared Channel). For identification of each UE, the MAC adds a radio network temporary identity (RNTI), identification information of a terminal, to a UE-ID field of the header and transmits it. The RNTI includes U-RNTI (UTRAN RNTI), C-RNTI (Cell RNTI) and DSCH-RNTI, so the MAC also adds a UE-ID type field indicating which RNTI is used and transmits it.
Identification of the dedicated logical channels is made through a C/T field. The reason is because, first, unlike other logical channels, several dedicated channels can be mapped to one transport channel, and second, the dedicated logical channel is processed in an MAC-d of a serving radio network controller (SRNC) and other logical channels are provided in an MAC-c/sh of a control radio network controller (CRNC). Dedicated logical channels mapped to one transport channel respectively have a logical channel identity that is used as a C/T field value. If only one dedicated logical channel exists in the transport channel, the C/T field is not used.
FIG. 5 illustrates MAC header information according to a mapping relation between the dedicated logical channel and the transport channel in accordance with the conventional art.
As shown in FIG. 5, the C/T field exists only when several dedicated logical channels (DCCH or DTCH) are mapped, ‘N’ means non-existence of a header, and ‘-’ means there is no mapping region. In addition, because the UE-ID field exists together with the UE-ID type field at the time, so it is simply indicated by UE-ID.
4. Measurement of Traffic Volume and Transport Channel Type Switching
In order to support the RRC in dynamically controlling a radio bearer, the MAC performs functions of measurement of a traffic volume and change of a type of a transport channel.
The measurement of traffic volume is performed on the transport channel. The MAC measures the size of the RLC buffer of every logical channel mapped to the transport channel at each TTI and adds the sizes to calculate a transport channel traffic volume. The traffic volume of a transport channel indicates the amount of data to be transmitted by that the transport channel. The MAC reports the measurement results to the RRC and the measurement results serve as a basis for the RRC to determine whether a corresponding transport channel may sufficiently transmit the measured amount of data.
The MAC reports the measurement result to the RRC. Unlike the measurement of the traffic volume performed at every TTI, the measurement result report is performed when a specific condition is satisfied unlike. The report type includes an event trigger method for reporting the measurement result when the measurement result exceeds a threshold value, and a periodical method for reporting the measurement result at every predetermined time.
Upon receiving the measurement result, the RRC determines whether a current transport channel is suitable for each radio bearer, and if the current transport channel is not suitable, the RRC commands the MAC to change a transport channel of a radio bearer. Namely, the transport channel type change is a function for effectively managing a resource of the transport channel by selectively using a suitable transport channel according to the amount of given data.
When a DCH is used, the efficiency of a coded-divided channel may be problematic and there may not be enough codes for use for data transmissions having burst characteristics that result in data being crowded at a specific time during a communication session. In order to solve this problem, several scrambling codes may be used. However, the complexity of a receiver may increase without increasing the efficiency of the code-divided channel.
The DSCH is a channel shared by several users transmitting dedicated control or traffic data. Several users may share one channel by performing code multiplexing. Therefore, the DSCH may be defined as a series of code sets.
Unlike the uplink, a code shortage occurs in the downlink because the number of codes one sector may have in one base station is limited due to a spreading factor. For a high transmission rate, a low spreading factor must be used, thereby reducing the number of physical channels.
Additionally, such data services generally have burst characteristics. Therefore, if one channel is continuously allocated to one service, codes cannot be used efficiently.
In order to solve these problems, a method in which one channel is shared by a plurality of users may be employed. In order to share one channel, code multiplexing is used. Code allocation is performed for every radio frame, for example time multiplexing.
The multimedia broadcast/multicast service (MBMS) will now be described.
The CBS has limitations. First, the maximum length of a CBS message is limited to 1230 octet. Therefore, a CBS message is not suitable for broadcasting or multicasting multimedia data. Second, since the CBS message is broadcast to every terminal in a specific cell, multicasting for providing a service to only a specific terminal group is not possible wirelessly. For these reasons, a new service called MBMS has been proposed.
The MBMS is a service for transmitting multimedia data such as audio, video or image data to plural terminals by using a unidirectional point-to-multipoint bearer service. The MBMS is divided into a broadcast mode and a multicast mode. That is, the MBMS is divided into an MBMS broadcast service and an MBMS multicast service.
1. Users receive a service announcement provided by a network. The service announcement indicates a list of services to be provided and provides related information to terminals.
2. The network sets a bearer for a corresponding broadcast service.
3. Users receive a service notification provided by the network. The service notification provides information related to broadcast data to be transmitted to terminals.
4. Users receive broadcast data from the network.
5. The network releases a bearer for a corresponding broadcast service.
The MBMS broadcast mode is a service for transmitting multimedia data to every user in a broadcast area. The broadcast area means a broadcast service available area. One or more broadcast areas may exist in one PLMN, one or more broadcast services can be provided in one broadcast area, and one broadcast service can be provided to several broadcast areas.
The MBMS multicast mode is a service for transmitting multimedia data only to a specific user group existing in a multicast area. The multicast area means a multicast service available area. One or more multicast areas can exist in one PLMN, one or more multicast services can be provided in one multicast area, and one multicast service can be provided to several multicast areas.
In the multicast mode, a user is requested to join a multicast group to receive a specific multicast service. At this time, the multicast group refers to a user group that receives the specific multicast service, and joining refers to a behavior of being admitted to the multicast group intending for receiving the specific multicast service.
1. A user subscribes to a multicast subscription group. Subscription involves establishing a relationship between a service provider and a user. A multicast subscription group is a group of users that have completed the subscription procedure.
2. Users that have subscribed to the multicast subscription group receive a service announcement provided by the network. The service announcement indicates a list of services to be provided and provides related information to terminals.
3. In order for a user that has subscribed to a multicast subscription group to receive a specific multicast service, the user must join a multicast group. A multicast group is a group of users that receive the specific multicast service. Joining a multicast group involves joining the users intending to receive the specific multicast service. Joining a multicast group is also referred to as MBMS multicast activation. Through MBMS multicast activation, a user may receive specific multicast data.
4. The network sets a bearer for a corresponding multicast service.
5. A user joining the multicast group receives a service notification provided by the network. The service notification provides information regarding multicast data to be transmitted to terminals.
6. Users receive multicast data from the network.
7. The network releases a bearer for a corresponding broadcast service.
MBMS data is transmitted from the RNC to a base station and to a terminal by using services of the PDCP layer, the RLC layer, the MAC layer and the physical layer positioned at the user plane of the UTRAN protocol. That is, the MBMS data transmitted from the core network (CN) is subjected to a header compression at the PDCP layer and transmitted as an RLC UM entity through an RLC UM SAP, and then, the RLC UM entity is transmitted to the MAC layer through the common traffic channel, the logical channel.
The MAC layer adds an MAC header to the received MBMS data and transfers it to the physical layer of the base station through the common transport channel. And then, the MBMS data undergoes coding and modulation in the physical layer and transmitted to the terminal through the common physical channel.
An MBMS RB, a radio bearer (RB) for the MBMS, serves to transmit user data of one specific MBMS service transferred from the core network to UTRAN to a specific terminal group. The MBMS RB is roughly divided into a point-to-multipoint RB and a point-to-point RB. In order to provide the MBMS service, UTRAN selects one of the two types of MBMS RBs. In order to select the MBMS RB, UTRAN recognizes the number of users of the specific MBMS service existing in one cell. UTRAN internally sets a threshold value, and if the number of users existing in a cell is smaller than the threshold value, UTRAN sets the point-to-point MBMS RB, whereas if the number of users existing in a cell is greater than the threshold value, UTRAN sets the point-to-multipoint MBMS RB.
The wireless system of the third generation partnership project (3GPP) proposes a downlink shared channel (DSCH) including a high speed downlink shared channel (HS-DSCH), particularly to support a packet data service.
In order for the DSCH to provide a multicast service, it should support the point-to-multipoint radio bearer, and at this time, the common logical channel such as CTCH or MTCH (MBMS Traffic Channel) should be mapped to the DSCH. In this respect, however, in the conventional art, because the DSCH transmits only data of the dedicated logical channel, a field for identifying a logical channel mapped to the DSCH is not added in the MAC header. Thus, when the common logical channel data is transmitted through the DSCH, in the case that the field indicating a type of the logical channel is not included in the MAC header in transmission of the DSCH, the terminal can not know which type of logical channel a data unit received through the DSCH belong to, and thus, there is a high possibility that a communication error occurs.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.